1. Field of Invention
The present invention relates to terrain following, terrain avoidance, and obstacle warning radar; and more particularly to a method and system for detecting radiant energy reflected by a wire located above the terrain being observed.
2. Discussion of Related Art
Terrain following, terrain avoidance, or obstacle warning radar system, referred to hereinafter collectively or individually as a terrain observation system, is typically mounted on aircraft and scan the earth's surface usually in the direction of aircraft travel for determining the elevation or range of the terrain, including buildings or other obstacles, from the radar platform.
Such systems may be either real aperture or synthetic aperture radar systems with pulses of high frequency and short duration and high repetition rate in order to provide a high resolution range for the observer. In such systems, it is common to employ pulse compression for obtaining a higher range resolution for the received echoes while permitting the transmission of longer pulses to provide more power to the radiated energy.
When the radiated energy of the transmitter strikes the surface of the earth, and other obstacles between the surface and the radar platform, the energy is reflected or scattered in all directions and with varying amplitudes which are a function of the shape of the object or radar cross-section, the beam angle, frequency, polarization of the return signal, and the reflection coefficient of the target material. The radar cross section of a target is the area intercepting the radiated energy or power, which produces an echo at the receiver corresponding to that from the target. In most instances, the radar cross-section is proportional to the physical area of the reflecting object.
Wire, which is located at various distances above the terrain, is unable to be reliably detected by the typical terrain observance system because it competes with a composite of reflecting elements within a unit of volume, whose size is determined by range resolution, and azimuth elevation beamwidth.
Wire provides a significant radar cross-section that is readily detected from other objects, and has maximum reflection at times when it is oriented perpendicular to the line-of-sight of the radar, or in other words parallel to the polarization of the transmitted beam. However, its reflectivity decreases rapidly as its angle of orientation changes from extending parallel to the polarization plane to an orientation orthogonal to the polarization plane, at which orientation it is effectively transparent to the radar beam.
For the most part, wire is usually strung parallel to the general contour of the terrain. This, of course, provides the same orientation relative to the terrain being observed. The actual orientation of wire relative to the radar platform, however, depends on the direction of its longitudinal axis relative to the direction of travel of the aircraft with respect to the earth's surface. Thus, the reduced radar cross-section of wire at angles that are not parallel to the polarization of the beam is many times too low relative to surrounding objects, to enable measurement of the wire height with the required accuracy.